Coating for internal surfaces of an airfoil and method of manufacture thereof

ABSTRACT

Disclosed herein is a method of coating, comprising providing an article having an internal passage therein to be coated; electrolytically applying a first layer that comprises chromium or a chromium alloy onto a surface of the internal passage; electrolytically applying a second layer comprising aluminum or an aluminum alloy onto the first layer; and heat treating the article to promote interdiffusion between the first layer and the second layer.

BACKGROUND

The present disclosure relates to a method of coating an internalsurface of a component to enhance hot corrosion resistance. Inparticular, the present disclosure relates to a method of coating theinternal surface of an airfoil to enhance hot corrosion resistance.

During operation of gas turbine engines, the temperatures of combustiongases may exceed 3,000° F., which is considerably higher than themelting temperatures of the metal parts of the engine, which are incontact with these gases. The metal parts that are particularly subjectto temperature extremes and degradation by the oxidizing and corrosiveenvironment, and thus require particular attention with respect tocooling, are the hot section components exposed to the combustion gases,such as blades and vanes used to direct the flow of the hot gases, aswell as other components such as shrouds and combustors.

High and low pressure turbine airfoils are manufactured from nickelbased super alloys. These components are protected against the hightemperature environment by a thermal barrier coating (TBC). However, theinternal surface of airfoil can be difficult to coat, therefore oftensusceptible to high temperature oxidation and material damage and lossin more corrosive environment.

Internal surface oxidation has been found to be responsible for airfoilperformance deterioration due to blockage of the air passages in thetrailing edge. This issue is particularly pronounced in countries whereair pollution leads to high SO₂ concentration and airborne particulatematters contains sulfate and phosphate in the atmosphere. Repairing theairfoil with internal oxidation damages involves rebuilding the wall andreplenishment of elements that can form thermally grown oxides forprotection. Traditional coating application techniques such as plasmaspray, cathodic arc, electron beam physical vapor etc., are not suitablefor coating the internal surface due to line of sight limitation.

SUMMARY

Disclosed herein is a method of coating, comprising providing an articlehaving an internal passage therein to be coated; electrolyticallyapplying a first layer that comprises chromium or a chromium alloy ontoa surface of the internal passage; electrolytically applying a secondlayer comprising aluminum or an aluminum alloy onto the first layer; andheat treating the article to promote interdiffusion between the firstlayer and the second layer.

In an embodiment, the electrolytically applying of the first layerand/or the electrolytically applying of the second layer is conductedvia electrodeposition.

In another embodiment, the electrodeposition is conducted viaelectroless deposition.

In yet another embodiment, the electrodeposition is conducted by usingconforming electrodes that traverse internal passages of the componentwithout contacting a surface of the article.

In yet another embodiment, the conforming electrode is an anode andwhere the article is a cathode.

In yet another embodiment, the conforming electrode is coated with aporous electrically insulating material.

In yet another embodiment, the electrolytically applying of the firstlayer comprises using an electrolyte that comprises a suspension ofchromium or nickel-chromium.

In yet another embodiment, the electrolytically applying of the secondlayer comprises using an electrolyte that comprises a suspension ofaluminum or an aluminum alloy.

In yet another embodiment, the first layer has a thickness of 50 to 100micrometers.

In yet another embodiment, the second layer has a thickness of 25 to 75micrometers.

In yet another embodiment, the heat treating is conducted at atemperature of 800 to 1600° C.

In yet another embodiment, the heat treating results in the formation ofa layer of thermally grown oxides that comprise alumina and chromiumoxide that is disposed on a layer of Ni—Cr/aluminide or a layer ofCr/aluminide.

In yet another embodiment, the Ni—Cr/aluminide layer or the Cr/aluminidelayer contains aluminum and chromium that vary in amount inversely withone another with distance from a surface of the article or from asurface of the thermally grown oxide layer.

In an embodiment, the article is an airfoil.

Disclosed herein too is an airfoil comprising an internal surface havingdisposed thereon a thermally grown oxide layer that comprises aluminaand chromium oxide; and a layer comprising Ni—Cr/aluminide orCr/aluminide disposed between the layer comprising alumina and chromiumoxide and the internal surface of the airfoil.

In yet another embodiment, the layer comprising Ni—Cr/aluminide orCr/aluminide contains aluminum and chromium that vary in amountinversely with one another with distance from the internal surface ofthe airfoil or from a surface of the thermally grown oxide layer.

In yet another embodiment, the thermally grown oxide layer has athickness of 2 to 7 micrometers.

In yet another embodiment, the layer comprising Ni—Cr/aluminide orCr/aluminide has a thickness of 50 to 100 micrometers.

In yet another embodiment, the layer comprising Ni—Cr/aluminide orCr/aluminide is obtained by thermally treating a first layer comprisingchromium or nickel-chromium that is disposed on the internal surface ofthe airfoil; and a second layer that contains aluminum or an aluminumalloy that is disposed on the first layer.

In yet another embodiment, the first layer and the second layer aredeposited electrolytically.

The foregoing features and elements may be combined in variouscombinations without exclusivity, unless expressly indicated otherwise.These features and elements as well as the operation thereof will becomemore apparent in light of the following description and the accompanyingdrawings. It should be understood, however, that the followingdescription and drawings are intended to be exemplary in nature andnon-limiting.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features will become apparent to those skilled in the art fromthe following detailed description of the disclosed non-limitingembodiments. The drawings that accompany the detailed description can bebriefly described as follows:

FIG. 1 is a schematic view of a component illustrated as a turbineblade;

FIG. 2 is an enlarged schematic cross-section of the internal surfacesof the turbine blade of FIG. 1;

FIG. 3 is a flow chart of a method for producing the protective coatingon an internal surface of the turbine blade; and

FIG. 4 is an exemplary schematic cross-sectional view of the method ofcoating the internal surface.

DETAILED DESCRIPTION

Disclosed herein is a protective coating for the internal surface of anairfoil that enhances hot corrosion resistance. The protective coatingcomprises multiple elements that can form an oxidation protectionstructure and compositions during coating preparation as well assubsequent high temperature operation. A multilayered compositeprecursor coating is treated to homogenize at least part of theresulting coating that comprises a metal oxide layer disposed on achromium aluminide layer or a nickel-chromium aluminide layer thatcontacts the internal surface of the airfoil.

Disclosed herein too is a method for coating the internal surface of anengine airfoil. This method entails disposing a first layer comprising ametallic nickel-chromium alloy (hereinafter “Ni—Cr alloy”) or chromiummetal on an internal surface of the airfoil, followed by a second layerthat comprises aluminum metal or a metallic aluminum alloy (hereinafterAl or Al alloys). The first layer contacts the internal surfaces of theairfoil while the second layer is disposed on and contacts the firstlayer. In an embodiment, both the first and the second layer areelectrolytically deposited. In an exemplary embodiment, the first layercomprising Ni—Cr and/or Cr and the second layer comprising Al and/or theAl alloy are applied by electrodeposition, electroless deposition, or acombination that comprises both electrodeposition and electrolessdeposition.

When electrodeposition is deployed, conforming electrodes (preferablyconforming anodes) that can conform to the convoluted passages of theairfoil are used to facilitate coating the inner surface of the airfoil.The use of conforming electrodes facilitates coating those innersurfaces of the airfoil which would otherwise be difficult to reach. Theproximity of the conforming electrodes to the inner surfaces of theairfoil provides adequate throwing power to achieve effective coverageof the inner surfaces with the coating.

Following the deposition of the first and the second layers, the innersurface of the airfoil is subjected to heat treatment at an elevatedtemperature that produces purification and densification of therespective coating layers in addition to homogenizing the compositionsin the two distinct layers. Reactions at high temperature allowsreactive element aluminum to interact with any impurities such as oxidesand chlorides incorporated during preceding depositions and results inremoving these impurities from the bulk coating or immobilizing them toimprove the high temperature resistance performance. The elevatedtemperature treatment produces a reaction between the chromium andaluminum to produce an aluminide diffusion layer. This diffusion layer(also termed a composite layer) restores and enhances high temperaturehot resistance of the inner surface of the airfoil. Both chromium andaluminum readily form thermally grown oxides when subjected to elevatedtemperatures, which are known to be protective under high temperature.The aluminide layer further provides an aluminum reservoir, whichretards the outward diffusion of substrate elements during service, thusincreasing life cycles of the airfoil and reducing maintenance andrepair.

FIG. 1 depicts a component article of a gas turbine engine such as aturbine blade or a turbine vane, and in this illustration is representedas a blade 20. The blade 20 is formed of any operable material, such as,for example, a nickel-base superalloy substrate. The blade 20 includesan airfoil 22 against which the flow of hot exhaust gas is directed.

The blade 20 is mounted to a turbine disk (not shown) by a dovetail 24that extends downwardly from the airfoil 22 and engages a slot on theturbine disk. A platform 26 extends laterally outwardly from the areawhere the airfoil 22 is joined to the dovetail 24. The airfoil 22 may bedescribed as having a root end 30 adjacent to the dovetail 24, and anoppositely disposed tip end 32 from the dovetail 24. A number ofinternal passages extend through the interior of the airfoil.

An exemplary depiction of the internal passages that lie in the interiorof the airfoil is depicted in the FIG. 2. The FIG. 2 depicts anexemplary embodiment, where conforming electrodes are placed in theinternal passages of the airfoil to facilitate electrodeposition of thefirst and the second layer. The electrodeposition will be detailedlater. As may be seen in FIG. 2, a number of internal passages 28 extendthrough the interior of the airfoil 22. As can be seen, these passagesare convoluted in shape as a result of which they are difficult toaccess. During service, a flow of cooling air is directed through theinternal passages 28, usually from the root end 30 toward the tip end32, to reduce the operational temperature of the airfoil 22.

With reference to FIGS. 2, 3 and 4, one exemplary non-limitingembodiment of a method 200 for coating the internal passages 28 isdisclosed. As noted above, FIG. 2 depicts the conformal anodes thattraverse the convoluted internal passages of the airfoil. The blade 20,which has a coating to be applied within the internal passages 28, andwhich serves as a cathode in the coating application method, receives aconforming wire anode 40 (FIG. 2) for coverage of the internal passages28 during electrodeposition or electroless deposition. The FIG. 3depicts a general method for coating the internal passages of theairfoil while FIG. 4 depicts an exemplary schematic diagram fordisposing the first and second layers on the internal surfaces and theformation of the diffusion layer after heat treatment.

With reference now to FIGS. 3 and 4, a method 200 for depositing thefirst layer and the second layer comprises first cleaning the internalsurface 300 (see FIG. 4) by degreasing it and cleaning it with an acid.This is shown in step 202 of FIG. 3. The cleaning removes impuritiessuch as sulfates, oxides, nitrates, and the like. This cleaning is alsotermed a pre-treatment step. In an embodiment, the pretreatment step mayinclude degreasing, rinsing, acid cleaning and a final rinse.

The nickel-chromium or chromium layer (the first layer 302 in FIG. 4) isthen disposed on the internal surface of the airfoil byelectro-deposition (using conforming electrodes) or by electrolessdeposition. (See step 204 in FIG. 3) While step 204 discusses the use ofconforming electrodes, this method is used only for electro-deposition.No electrodes are used for electroless deposition. The first layer 302comprises chromium or nickel-chromium and has a thickness of 5 to 200micrometers, preferably 50 to 100 micrometers after the deposition.

When electro-deposition is used, the conforming electrodes are shaped toconform approximately to the shapes of internal passages of the airfoil.The conforming electrode is preferably an anode and conforms to thepassages whilst not contacting the airfoil internal surface.

The electroplating solution comprises chromium compounds, preferablyenvironmentally benign trivalent chromium species and nickel compounds.Traces of other metals that may be present in the electrolyte includeone or more of Zr, Hf, Ti, Ta, Si, Ca, Fe, Y and Ga.

When nickel and chromium are used in the first layer, the weight ratioof nickel to chromium is from 5:95 to 95:5, preferably 10:90 to 90:10,and more preferably 20:80 to 80:20.

Following the formation of the first layer, the coated internal surfacemay be optionally rinsed to remove traces of undesirable surfaceimpurities. It may be then subjected to optional acid activation.

The primary difference between electrodeposition and electrolessdeposition is how the Ni, Cr, or Ni—Cr alloy are deposited. Electrolessdeposition relies on catalyzed reaction occurring on the surface of asubstrate to reduce metal ions to metallic deposits. Oxidation of thereducing agent and the reduction of metal ions occurs at the same siteon the substrate during electroless deposition, whereas reduction ofmetal ions (cathodic reaction) and oxidation of other substances (anodicreaction) take place on the cathode and anode, respectively. Theelectrodeposition is driven by an external current. Because electrolessdeposition does not reply upon current flow through an anode and acathode, electroless deposition can proceed on the entire surfaceregardless of geometrical constraints. Electroless deposition is a trulynon-line-of-sight deposition if the surface is pre-treated uniformly.

During electro-deposition with the conforming electrodes, both the bladeand the conforming anode (e.g., wire or ribbon) are located in anelectrolyte (not shown). The plating solution includes an electrolytecontaining the ionic species of the coating composition to be deposited.The electrical field formed between the blade and the conforming wireanode permits the coating material dissolved in the electrolyte to bedeposited onto the surface of the internal passages.

The conforming wire anode is insulated from direct contact with theinternal passages by a porous sheath 42 (See FIG. 2). In other words,the conforming wire anode 40 is shaped to extend into the internalpassages 28 but insulated from direct contact by the porous sheath 42.The close proximity of the conforming wire anode 40 to the internalpassages 28 assures that the variation associated with the platingelectrolyte resistance is relatively small which results in uniformcurrent distribution. The conforming wire anode 40 extends into theinternal passages 28 to improve the throwing power of the platingelectrolyte. The article may be held in a fixture 50 at its base 34during the electrodeposition.

The porous sheath generally comprises a non-electrically conductingporous polymer such as a polyolefin, polyvinyl acetate, polyvinylchloride, polyimide, polysiloxane, polystyrene, polymethylmethacrylate,or a combination thereof.

Throwing power of the plating electrolyte can be specifically engineeredby, for example, an increase to the charge transfer resistanceassociated with metal deposition with an increased conductivity of theplating electrolyte. This, in turn, increases coverage of the substrateto be plated by the Ni—Cr coating or Cr coating 302 (see FIG. 4). Byincreasing the throwing power, internal passages 28A (see FIG. 2), whichdo not even contain the conforming wire anode 40, can be coated. Thethickness of the first layer can range from 50 to 100 micrometers beforeheat treatment and the partially homogenized composite coating with adiffusion zone can range from 50 to 150 micrometers.

Following the formation of the first layer, the coated surface is thenoptionally rinsed, acid cleaned, and subjected to a final rinse (see 206in FIG. 3).

Next, the blade 20 (see FIGS. 1 and 2) is again mounted to receive theconforming wire anode 40 (see step 208 in the FIG. 3). An aluminum oraluminum alloy second layer is applied by electrodeposition onto the Cror Ni—Cr first layer (see step 208 in the FIG. 3; see also layer 304 inFIG. 4).

The electrolyte solution of the deposition of aluminum or the aluminumalloy generally comprises non-aqueous aluminum electrolytes such aschloroaluminate ionic liquids. The second layer 304 (see FIG. 4)comprises aluminum or aluminum alloys and has a thickness of 5 to 200micrometers, preferably 25 to 75 micrometers after the deposition.

Following the deposition of the second layer 304, the blade may beoptionally rinsed to remove traces of surface impurities and unusedelectrolyte (See step 210 in FIG. 3.). The blade with the first layer302 and the second layer 304 is then subjected to a heat treatment at atemperature of 800 to 1600° C., preferably 1000 to 1500° C. for periodof 2 to 18 hours, preferably 4 to 8 hours (See step 212 in FIG. 3.). Theheat treatment may occur in the presence of an inert atmosphere such asargon, nitrogen, carbon dioxide, or the like, or a combination thereof.

The heat treatment purifies and densifies the coatings 302 and 304 by ahigh temperature reaction to form a thermally grown oxide (TGO) layer308 that comprises alumina (Al₂O₃) and chromium oxide (Cr₂O₃) atop apurified Ni—Cr/aluminide or Cr/aluminide layer 306 (see FIG. 4). Othermethods of growing alumina (Al₂O₃) and chromium oxide (Cr₂O₃) atop thepurified Ni—Cr/aluminide or Cr/aluminide layer 306 are detailed in U.S.patent having Ser. No. 15/604,950 to Opalka et al., filed on May 25,2017, the entire contents of which are hereby incorporated by reference.

The thermally grown oxide layer 308 has a thickness of 0.5 to 25micrometers, preferably 2 to 7 micrometers. The Ni—Cr/aluminide layer ofthe Cr/aluminide layer 306 has a thickness of 5 to 200 micrometers,preferably 50 to 100 micrometers.

This heat treatment reduces impurities present in the Ni—Cr deposits toinduce a reaction at a high temperature that uses aluminum as thescavenger. This leads to the formation of a diffused protective aluminaon the surfaces of the internal passages of the airfoil. This treatmentalso facilitates densifying of the Ni—Cr or Cr first layer whichprevents the outward diffusion of substrate elements during service.

In an embodiment, the heat treatment results in the diffusion ofaluminum from the second layer 304 into the first layer 302 thatcontains either chromium or nickel-chromium (See FIG. 4). This diffusionresults in the formation of the purified Ni—Cr/aluminide layer or aCr/aluminide layer 306 (see FIG. 4). The purified Ni—Cr/aluminide layerof the Cr/aluminide layer 306 in the FIG. 4 has a gradient of aluminumand chromium that varies inversely with one another. The aluminumdecreases in amount as the distance from the thermally grown oxideincreases while the chromium increases in amount as the distance fromthe internal surface of the airfoil increases. The ratio of aluminum tochromium varies inversely as the distance from the thermally grown oxideincreases, or alternatively, as the distance from the internal surfaceof the airfoil increases. This gradient may be a linear gradient, acurvilinear gradient or a step gradient.

The Ni—Cr/aluminide zone or the Cr/aluminide zone 306 (see FIG. 4)serves as a reservoir for both aluminum and chromium that replenishesthe thermally grown oxide layer 308 that comprises alumina (Al₂O₃) andchromium oxide (Cr₂O₃). As the thermally grown oxide layer 308 getsdepleted with usage, aluminum and chromium from the Ni—Cr/aluminide zone306 or the Cr/aluminide zone 306 diffuses towards the thermally grownoxide layer 308 to form additional alumina and chromium oxide, thuspreventing damage to the internal surface of the airfoil. This increasesthe life cycle of the component and also of the engine.

The electrodeposition process is advantageous in that it is low cost andis performed with elements and processes that are chemically benign.

The use of the terms “a,” “an,” “the,” and similar references in thecontext of description (especially in the context of the followingclaims) are to be construed to cover both the singular and the plural,unless otherwise indicated herein or specifically contradicted bycontext. The modifier “about” used in connection with a quantity isinclusive of the stated value and has the meaning dictated by thecontext (e.g., it includes the degree of error associated withmeasurement of the particular quantity). All ranges disclosed herein areinclusive of the endpoints, and the endpoints are independentlycombinable with each other. It should be appreciated that relativepositional terms such as “forward,” “aft,” “upper,” “lower,” “above,”“below,” and the like are with reference to normal operational attitudeand should not be considered otherwise limiting.

Although the different non-limiting embodiments have specificillustrated components, the embodiments of this invention are notlimited to those particular combinations. It is possible to use some ofthe components or features from any of the non-limiting embodiments incombination with features or components from any of the othernon-limiting embodiments.

It should be appreciated that like reference numerals identifycorresponding or similar elements throughout the several drawings. Itshould also be appreciated that although a particular componentarrangement is disclosed in the illustrated embodiment, otherarrangements will benefit herefrom.

Although particular step sequences are shown, described, and claimed, itshould be understood that steps may be performed in any order, separatedor combined unless otherwise indicated and will still benefit from thepresent disclosure.

The foregoing description is exemplary rather than defined by thelimitations within. Various non-limiting embodiments are disclosedherein, however, one of ordinary skill in the art would recognize thatvarious modifications and variations in light of the above teachingswill fall within the scope of the appended claims. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practiced other than as specifically described. For that reasonthe appended claims should be studied to determine true scope andcontent.

What is claimed is:
 1. A method of coating, comprising: providing anarticle having an internal passage therein to be coated;electrolytically applying a first layer that comprises chromium or achromium alloy onto a surface of the internal passage; electrolyticallyapplying a second layer comprising aluminum or an aluminum alloy ontothe first layer; and heat treating the article to promote interdiffusionbetween the first layer and the second layer; where the electrolyticallyapplying of the first layer and the second layer is conducted by using aconforming electrode that traverses internal convoluted passages of thearticle without contacting a surface of the article; where the internalconvoluted passages are insulated from the conforming electrode by aporous sheath that is disposed on the conforming electrode; and whereinthe porous sheath comprises an electrically insulating material thatcomprises a polyolefin, a polyvinyl acetate, a polyvinyl chloride, apolyimide, a polysiloxane, a polystyrene, a polymethylmethacrylate, or acombination thereof.
 2. The method of claim 1, wherein theelectrolytically applying of the first layer and/or the electrolyticallyapplying of the second layer is conducted via electrodeposition.
 3. Themethod of claim 1, where the conforming electrode is an anode and wherethe article is a cathode.
 4. The method of claim 1, where theelectrolytically applying of the first layer comprises using anelectrolyte that comprises a suspension of chromium or nickel-chromium.5. The method of claim 1, where the electrolytically applying of thesecond layer comprises using an electrolyte that comprises a suspensionof aluminum or an aluminum alloy.
 6. The method of claim 1, where thefirst layer has a thickness of 50 to 100 micrometers.
 7. The method ofclaim 1, where the second layer has a thickness of 25 to 75 micrometers.8. The method of claim 1, where the heat treating is conducted at atemperature of 800 to 1600° C.
 9. The method of claim 1, where the heattreating results in the formation of a layer of thermally grown oxidesthat comprise alumina and chromium oxide that is disposed on a layer ofNi—Cr/aluminide or a layer of Cr/aluminide.
 10. The method of claim 9,where the Ni—Cr/aluminide layer or the Cr/aluminide layer containsaluminum and chromium that vary in amount inversely with one anotherwith distance from a surface of the article or from a surface of thethermally grown oxide layer.
 11. The method of claim 1, where thearticle is an airfoil.